Solar extreme solar proton events (SPEs) form important radiation hazards for the modern technological society. The strongest directly observed SPE took place on 23-Feb-1956 as an up to 5000 % increase of the count rate of ground-based neutron monitors. It was characterized by a very hard energy spectrum and strong particle fluence. On the other hand, as known from indirect proxies (cosmogenic isotopes), several extreme events, one–two orders of magnitude stronger, occurred during the past millennia. In order to study past events, a reference scale needs to be made. The SPE of 23-Feb-1956 is often used as such a reference. Thanks to the recent developments in the methodology of SPE analysis, the spectrum of fluence of the reference event have been revisited and re-assessed with higher precision. Here we present an estimate of the sensitivity of
the cosmogenic-isotope method to detect extreme SPEs in the past. It is shown that the modern accuracy of the cosmogenic-isotope method to SPEs is insufficient, by an order of magnitude for any single isotope record, to detect the reference event but can resolve events a factor 3–4 stronger using a multi-proxy method. This provides a solid basis for research in the field of extreme events, both for fundamental science, namely solar and stellar physics, and practical applications, such as the risk assessments of severe space-based hazards for a modern technological society.